Olefin-paraffin separations represent a class of most important and also most costly separations in the chemical and petrochemical industry. Cryogenic distillation has been used for over 60 years for these separations. They remain to be the most energy-intensive distillations because of the close relative volatilities. For example, ethane-ethylene separation is carried out at about -25.degree. C. and 320 psig in a column containing over 100 trays, and propane-propylene separation is performed by an equally energy-intensive distillation at about -30.degree. C. and 30 psig.
Olefins are, generally, produced via catalytic cracking processes. Such processes produce refinery-grade olefins (65-70 purity). Currently. refinery grade olefins are further separated and purified using distillation columns to produce polymer-grade olefins (99.5% purity) or chemical-grade olefins (95% purity). Frequently two distillation columns must be employed. Each distillation step is expensive and energy intensive, and even incremental gains in purity greatly increase the costs of the olefin products.
Current theories for improving the economics of olefin separations and purifications suggest that a hybrid separation process be utilized. In addition to the conventional distillation step, some other chemically specific process would be utilized to enhance olefin purity for greatly reduced costs. Processes that have been suggested as potentially being amenable to the hybrid approach include: 1) a facilitated transport membrane using a chemically-specific complexing agent; 2) absorption/stripping with a chemical solvent; and 3) adsorption/desorption on a solid support.
Rationale for use of hybrid olefin separation processes is as follows: conventional separation technology can only achieve a certain level of separation per stage. This level of separation is not a constant for each stage. As higher purity levels are required, the number of stages increases rapidly. This also means a dramatic increase in the costs for additional processing equipment. On the other hand, a separation step using reversible chemical complexation obtains improved selectivity at the same time that the driving force of conventional olefin separation processes decreases. Although this is not intuitive, it occurs because there is a large excess of complexing agent present and the selective reaction becomes very efficient. The hybrid process, therefore, typically combines a conventional separation process to achieve a certain level of purity and follows it with a separation step using reversible chemical complexation to "polish" or further purify the desired product. See, Haggin, Chem. & Eng. News., pg. 23-24 Feb. 25, 1991.
Previous reversible binding systems have used a complexation agent that contains a metal ion that has an affinity for the unsaturated hydrocarbon to be separated. Processes for separating unsaturated hydrocarbons from a gaseous hydrocarbon mixture by contacting the mixture with an aqueous solution of heavy metal salts capable of forming reversible complexes with the unsaturated hydrocarbons, whereby the unsaturated hydrocarbons are complexed and extracted by such aqueous solution, are known. Water-soluble silver or copper salts or complexes have been reported as being capable of forming reversible complexes with unsaturated hydrocarbons in such processes. Silver ion, Ag(I), is known to be especially useful for the separation of olefins. However, at least some of such salts or complexes are reported to be adversely affected by certain materials that are frequently present in the gaseous mixtures from which the unsaturated hydrocarbons are to be separated or to form complexes with alkenes or alkynes that are thermally unstable.
AgNO.sub.3 reacts, irreversibly, with sulfur compounds. Copper compounds are very susceptible to reactions with oxygen. water or sulfur compounds. Such reactions lead to very poor lifetimes in operation or, in the alternative, lead to the higher costs required for removing the compounds prior to separation and purification. Similar problems exist with acetylene (when purifying ethylene) and propyne (when purifying propylene). Again, the alkyne impurities increase costs either by greatly reducing the life of the complexing agent, or by the mechanism required to remove the alkyne prior to the separating process utilizing the complexing agent.
In a few instances, the ability of metal ions to bind reversibly to olefins has been utilized in olefin separation and purification systems. For example, olefin adducts of Ag(I) ion have been used in chromatographic systems for the separation of olefins. More recently, aqueous silver nitrate solutions have been used to separate ethylene or propylene from purified multicomponent gas streams. See, U.S. Pat. No. 4,174,353 of Marcinkowsky et al. A major concern when using the silver/olefin adduct chemistry, is that the silver ion forms a complex with acetylene which is explosive when dry and rigorous methods must be employed to remove acetylene from any gas stream that will come in contact with the silver ion. A further problem associated with silver/olefin separation schemes is that the silver ion is rapidly poisoned by H.sub.2 S, a common impurity in gas associated with the thermal cracking of hydrocarbons.
Thus, it is highly desirable to develop improved complexing agents for use in the aforesaid processes for separating unsaturated hydrocarbons from hydrocarbon mixtures containing them. Potential which molybdenum-sulfide dimers have as complexation agents in part of a hybrid olefin separation process has been recognized. See, for example, U.S. Pat. Nos. 5,391,791 in the name of Mary Rakowski Dubois, Richard Noble and Carl Koval; 5,414,194; and 5,430,225 which describe molybdenum sulfide dimer compounds and their use in olefin separation and acetylene removal processes. The dimers act as complexation agents for the olefins and acetylenes wherein the sulfide ligands of the molybdenum-sulfide dimers are believed to act as the site of olefin binding. The binuclear sulfide bridged molybdenum dimers have the general formula EQU (C.sub.5 H.sub.5 Mo).sub.2 (.mu.-S).sub.4-x (.mu.-SR).sub.x !.sup.n
where x is 0-3 and n is 0, +1, or -1. In addition, this structure can be modified in a limited number of ways, such as substitutions that can be made to the alkanedithiolate moiety R or to the cyclopentadienyl moiety C.sub.5 H.sub.5, (or Cp) in order to enhance the water solubility of the dimer or to introduce chemically reactive ligands that can be used to incorporate the dimer within the matrix of polymeric materials, such as membranes. The patents define suitable water soluble molybdenum-sulfide dimers as including all compounds that contain molybdenum and sulfur and that are capable of forming chemically or thermally-reversible alkanedithiolate or alkenedithiolate complexes that have significant solubility in water or aqueous systems.
In contrast to the use of metal ion chemistry in olefin separation schemes, the molybdenum-sulfide dimers are unaffected by the presence of H.sub.2 S. In addition, the inventors of the present invention have also described how the ability of the molybdenum-sulfide dimers to bind and to subsequently reduce alkynes may be used in alkyne removal processes.
The dimeric complexation agents are disclosed as being useful in separation processes that include liquid/liquid, gas/liquid, liquid/solid and gas/solid separation procedures that are familiar to those skilled in the art. Separations by the use of the molybdenum-sulfide dimers that are disclosed include the separation of olefins from paraffins (for example, ethylene from ethane and propylene from propane), the separation of olefins (for example, ethylene from propylene), the separation of olefin isomers (for example, cis-2-butene from trans-2-butene), the separation of olefins from alkynes (for example, ethylene from acetylene and propylene from propyne), and the removal of alkynes from a gaseous hydrocarbon feed stream and the catalytic reduction of alkynes.
The molybdenum sulfide dimers disclosed in the aforesaid Dubois et al. patents were synthesized in accordance with the procedures of Dubois et al., J. Am. Chem. Soc., Volume 101, pages 5245-5252 (1979); Dubois et al., J. Am. Chem. Soc., Volume 102, page 7456 (1980); Dubois et al., Inorg. Chem., Volume 20, pages 3064-3071 (1981); M. McKenna et al., J. Am. Chem. Soc., Volume 105, pages 5329-5337 (1983); and J. Birnbaum et al., Organometallics, volume 10, pages 1779-1786 (1991). However, generally such syntheses are lengthy and complicated and afford a low yield of the desired product. Consequently, it is highly desirable to develop alternative processes for synthesis of molybdenum sulfide dimers that could be used as suitable complexation agents for the selective and reversible complexation of unsaturated hydrocarbons in the aforesaid processes for separating unsaturated hydrocarbons from multi-component gaseous mixtures containing them.
It is therefore a general object of the present invention to provide an improved process for synthesis of molybdenum sulfide dimers that affords a relatively uncomplicated synthesis while retaining their aforesaid desirable features.
More particularly, it is an object of the present invention to provide improved processes that substantially reduce the time required for preparation of the desired complexation agents which are soluble and stable in water and that selectively forms a reversible complex with an unsaturated hydrocarbon that is also soluble and stable in water.
It is another object of the present invention to provide improved processes that make more efficient use of reactants required to synthesis desired molybdenum sulfide dimers and thereby increase yields.
It is a further object of the present invention to provide an improved aforesaid process that employs a complexation agent whose stability and complexation activity are not adversely affected by other materials that are frequently present in the multi-component gaseous mixtures from which an unsaturated hydrocarbon is to be separated.
Other objects and advantages of the present invention will become apparent upon reading the following description and appended claims.